The good news is that smart people are working on it -- including Apple. The company recently posted a job listing for an engineer with experience with solar power.

Apple's job opening could be unrelated to any project that even explores solar power -- the expertise required could be applied to other thin-film technologies. But it's clear that Apple is interested in -- and working on -- solar smartphone technology.

In February, Apple filed a patent for a method of integrating solar panels into a mobile touchscreen. The company has older patents for solar gadgets as well.

Forget about that. It's not going to happen. Solar technology almost certainly won't be built into an iPhone within the next two years.

And if such a phone were to arrive on the market, it's very unlikely to be totally solar powered. Solar panels on phones might extend battery life, but phones will still have to be charged by plugging them in or via wireless charging.

But never say never. In fact, it's very likely that advances in thin solar paneling will make that technology irresistible for extending battery life beyond what batteries themselves can achieve.

Why solar power failed on Android phones

Two low-cost solar-powered Android phones emerged in 2011.

Two years ago, Samsung announced a $350 solar phone called the Replenish, which was advertised as environmentally friendly. (You can still buy the phone for about $99 at Walmart or even less on other discount electronics websites.)

The upside: The Replenish is made from about one-third post-consumer recycled plastic. A solar panel on the battery cover extends battery life when charged in sunlight.

The downside: It's an obsolete throwback that runs on Android 2.2 (Froyo) and has a physical keyboard, a small screen and a 2-megapixel camera. And the solar panel is pretty weak: An hour of charging gives you 20 minutes of talk time. Better than nothing, I guess.

The other solar-powered Android phone was a device called the Umeox Apollo, which launched in February 2011. Counterintuitively, that phone was targeted at the low end of the market. It had a tiny screen, a lousy camera and other limitations. And it didn't gain any significant mind- or market share. That may be why its Chinese maker dropped any mention of the phone from its website.

Between 2006 and 2011, a handful of other solar-powered Android phones emerged; all of them failed in the market.

The failed solar Android products remind me of the failed fingerprint sensor smartphones. Motorola built a fingerprint scanner into its first Atrix phone. The Toshiba Portege G900 and G500 phones had them, too. Hitachi's Japan-only W51H included a fingerprint reader. And there were others. None of them were popular with customers.

All these efforts failed for the same reason the Android solar phones failed: The technology wasn't ready for prime time. The implementations were horribly ugly.

I'm hoping that Apple will eventually do for solar phones what it did for fingerprint readers: Wait for the right technology, then integrate it into a phone in a way doesn't destroy the aesthetics of the device.

Still, there are some interesting Android projects. Take the Earl, for example. It's a $300 solar-powered Android tablet for hikers and campers. It's being self-crowd-funded and has far exceeded its fundraising goals.

Unlike most solar phones and tablets, which seek merely to extend battery life a little bit, the Earl is designed to run entirely on solar power, with no need to ever be plugged into an outlet. It achieves this feat by using a low-energy 1024x768 black-and-white E-ink touchscreen. Five hours of charging gives you 20 hours of use, according to the company.

The tablet has various features that are useful for survival in the great outdoors, including GPS, a thermometer, humidity and barometric pressure sensors, and a compass. It also has a built-in walkie-talkie feature and a radio that can pick up AM, FM and shortwave signals.

Why solar fails on phones

Despite huge demand for longer battery life on smartphones, the challenges to achieving that with solar technology are many. There's a simple reason for that: Smartphones are small; they have only a tiny bit of surface area for collecting light. On top of that, they're energy hogs, with power-gobbling components such as touchscreens, various types of sensors and powerful processors.

You'll note that the Earl device achieves solar self-sufficiency with a tablet-size surface area and a screen that's about the same quality as the ones on older Kindle models.

Smartphones can't be big enough and low-energy enough to viably use solar power and still succeed in the market as popular gadgets. That's especially true because apps are increasingly power- and resource-hungry. Many apps constantly ping for connectivity and location data and make other energy-intensive requests. Games tax graphics and processing hardware.

Still, there's hope. UCLA eggheads are working on a see-through film that functions as a solar panel. This would theoretically enable touchscreens to harvest energy from light. The technology is a long way from being ready for widespread use. It can currently convert only about 7.3% of the energy it receives into usable electricity.

Researchers at several other universities and companies around the world are also chipping away at the problem of integrating solar power into smartphones.

The Chinese telephone giant TCL Communication is reportedly working with a French solar company called Sunpartner to integrate solar panels into smartphone screens. Sunpartner's Wysips technology involves an ultra-thin transparent photovoltaic layer that sits under a touchscreen.

Despite the apparent limitations, I'm very bullish on the use of solar power for smartphones.

Why (and how) your smartphone

will eventually be solar-powered

I believe it's inevitable that within a few years, we'll never have to plug in our smartphones or think about power. Here's how I think that's going to happen.

First, battery technology will keep improving slowly, and cells will charge faster and last longer than today's batteries of the same size.

Second, I believe it's only a matter of time before smartphones have solar panels covering both the front and the back, using different technologies. This will enable your phone to soak up energy no matter which side is facing up when you put it down.

Third, wireless charging technology will take over. Tables, lamps and desks will all have wireless charging capabilities, so phones will pick up juice whenever they're placed on those surfaces.

Fourth, the wearable technology revolution will reduce our use of smartphones. In fact, as a daily Google Glass user, I find that I'm using Glass to check the time, for notifications, to reply to emails and do other run-of-the-mill tasks as my phone spends more time in a deep sleep.

But the biggest gains will come from improvements in power management. Motorola's new Moto X phone -- a device that I also use every day -- is a marvel in power management. It achieves this by using dedicated, low-power processing (originally developed for smartwatches) to listen for voice commands and keep tabs on the phone's location and orientation. It also has an AMOLED screen and is equipped with Motorola's Active Display technology. This combination eliminates the need for users to fire up the full screen and phone for common tasks like checking the time and checking for notifications.

The Moto X is just one example of how an unglamorous aspect of smartphone design can deliver significant improvements: Ongoing advances in power management may one day yield a phone we never have to charge.

Yes, solar-powered smartphones are coming. But by the time they arrive, the technology won't be a big deal. Our phones will use multiple technologies, including solar, that will work together to eliminate the need to ever plug them into the wall.

Self-charging smartphones will be the second most convenient aspect of our lives in the years ahead -- right after commuting to work via jetpack.